Not applicable.
This invention relates generally to computed tomography (CT) imaging and, more particularly, to methods and apparatus for positioning an X-ray beam in a multi-slice CT imaging system.
In at least one known computed tomography (CT) imaging system configuration, an X-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The X-ray beam attenuated by the object impinges upon an array of radiation detectors having first and second array ends. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the X-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired to produce a transmission profile.
In known third generation CT systems, the X-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the X-ray beam intersects the object constantly changes. A group of X-ray attenuation measurement, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the X-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two-dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
In a multi-slice system including two or more rows of detector elements extending within the x-y plane between the first and second array ends, movement of an X-ray beam penumbra over detector elements having dissimilar response functions can cause signal changes resulting in image artifacts. Opening system collimation to keep detector elements in the X-ray beam umbra can prevent artifacts but increases patient dosage. Known CT imaging systems utilize a closed-loop z-axis tracking system to position the X-ray beam relative to a detector array. It would be desirable to provide a closed-loop system that operated during patient scanning to maintain the X-ray beam penumbra in the Z-axis at a position relative to a detector array row edge to minimize patient dosage, but far enough away from the edge to reduce artifacts.
One solution has been to, during data acquisition, dynamically track the Z-axis position of the penumbra and umbra with respect to array rows at a single end of the detector array and then adjusts the X-ray beam to maintain the relative position of the beam with respect to the rows. While this solution has managed to reduce artifacts and radiation during acquisition, this solution still has at least one important shortcoming that has hampered effectiveness. Specifically, this solution is based on the assumption that the entire X-ray beam moves identically as a function of beam fan angle. In other words, this solution assumes that the Z-axis locations of the penumbra and umbra at the single tracking end is identical to the Z-axis locations at the array end opposite the tracking end and is also identical at every point between the first and second array ends.
This assumption, while valuable, is not entirely accurate. As known in the art, twisting movements referred to generally as dynamic skew that occur during gantry rotation cause the X-ray beam to move differently in Z as a function of the beam fan angle. The disparate movement in Z has been shown to cause positioning errors of up to 0.3 mm on the detector array at isocenter even where tracking and dynamic beam adjustments maintain the relative position of the beam at the tracking end. This dynamic skew error prohibits virtually any penumbra from being used for data acquisition without the risk of generating artifacts and hence restricts ability to minimize dose reduction.
U.S. Pat. No. 5,299,250 entitled xe2x80x9cComputer Tomography Apparatus With Compensation For Focus Migration By Adjustment Of Diaphragm Positionxe2x80x9d which issued on Mar. 29, 1994 describes one system wherein detectors at opposite ends of a detector array are used to identify Z-axis beam position at opposite ends of the array. Thereafter a collimator configuration is modified to maintain relative beam-detector position. While describing an advantageous solution to the dynamic skew problem, this patent fails to teach or suggest an optimal method or apparatus for determining beam location with respect to the detector at the tracking ends, how the two Z-axis positions are combined to adjust beam position and how the beam is controlled when one of the Z-axis detectors is blocked.
There is therefore provided, in one embodiment, a method for positioning an X-ray beam on a multi-slice detector array of an imaging system in which the detector array has rows of detector elements and is configured to detect X-rays in slices along a Z-axis. The method includes the steps of comparing data signals representative of X-ray intensities received from a first detector subset in each row of a detector row subset to generate a first position command, the first detector subset located proximate a first array end, comparing data signals representative of X-ray intensities received from a second detector subset in each row of the detector row subset to generate a second position command, the second detector subset located proximate a second array end opposite the first array end and positioning the X-ray beam in accordance with a result of the comparisons.
The above described embodiment and systems performing this method periodically adjust the X-ray beam position to maintain the beam penumbra at a minimal distance from the detector array edge, so that patient dosage is minimized and imaging artifacts are reduced.